I went over the Usage Manual for trying to configure Point & Click so that I can use it with Atom (as much as I like Frescobaldi, its poor performance with large scores makes it unusable after a certain number of lines of code). The usage manual
http://lilypond.org/doc/v2.20/Documentation/usage/configuring-the-system-for-point-and-click says to "simply" write this line
export LYEDITOR=atom, but doesn't say where. After that I suppose I can go to the GNOME 3 steps and follow them, but since those steps don't mention any particular editor, I guess I should first know what to do with said export line.
Martín.